www.pudn.com > MovingImage.rar > MovingImage.java

/*
 * MovingImage.
 */

import java.awt.*;
import java.applet.*;

/* 
 * This applet moves one image in front of a background image.
 * It eliminates flashing by double buffering.
 */

public class MovingImage extends Applet implements Runnable
{
    int frameNumber = -1;
    int delay;
    Thread animatorThread;

    Dimension offDimension;
    Image offImage;
    Graphics offGraphics;

    Image stars;
    Image rocket;

    public void init()
	{
        String str;
        int fps = 10;

        //How many milliseconds between frames?
        str = getParameter("fps");
        try
		{
            if (str != null)
			{
                fps = Integer.parseInt(str);
            }
        } catch (Exception e) {}
        delay = (fps > 0) ? (1000 / fps) : 100;

        //Get the images.
        stars = getImage(getCodeBase(), "starfield.gif");
        rocket = getImage(getCodeBase(), "rocketship.gif");
    }

    public void start()
	{
        //Start animating!
        if (animatorThread == null)
		{
            animatorThread = new Thread(this);
        }
        animatorThread.start();
    }

    public void stop()
	{
        //Stop the animating thread.
        animatorThread = null;

        //Get rid of the objects necessary for double buffering.
        offGraphics = null;
        offImage = null;
    }

    public void run()
	{
        //Just to be nice, lower this thread's priority
        //so it can't interfere with other processing going on.
        Thread.currentThread().setPriority(Thread.MIN_PRIORITY);
 
        //Remember the starting time.
        long startTime = System.currentTimeMillis();

        //This is the animation loop.
        while (Thread.currentThread() == animatorThread)
		{
            //Advance the animation frame.
            frameNumber++;

            //Display it.
            repaint();

            //Delay depending on how far we are behind.
            try
			{
                startTime += delay;
                Thread.sleep(Math.max(0, 
                                      startTime-System.currentTimeMillis()));
            } catch (InterruptedException e) {
                break;
            }
        }
    }

    public void update(Graphics g)
	{
		paint(g);
	}

    public void paint(Graphics g)
	{
        Dimension d = size();

        //Create the offscreen graphics context, if no good one exists.
        if ( (offGraphics == null)
          || (d.width != offDimension.width)
          || (d.height != offDimension.height) )
		  {
            offDimension = d;
            offImage = createImage(d.width, d.height);
            offGraphics = offImage.getGraphics();
        }

        //Erase the previous image.
        offGraphics.setColor(getBackground());
        offGraphics.fillRect(0, 0, d.width, d.height);
        offGraphics.setColor(Color.black);

        //Paint the frame into the image.
        paintFrame(offGraphics);

        //Paint the image onto the screen.
        g.drawImage(offImage, 0, 0, this);
    }

    void paintFrame(Graphics g)
	{
        Dimension d = size();
        int w;
        int h;

        //如果背景图象已经完全装入，就将它画出来
        w = stars.getWidth(this);
        h = stars.getHeight(this);
        if ((w > 0) && (h > 0))
		{
            g.drawImage(stars, (d.width - w)/2,
                        (d.height - h)/2, this);
        }

        //如果前景图象已经完全装入，就将它画出来
        w = rocket.getWidth(this);
        h = rocket.getHeight(this);
        if ((w > 0) && (h > 0))
		{
            g.drawImage(rocket, ((frameNumber*5) % (w + d.width)) - w,
                        (d.height - h)/2, this);   //图象的水平显示位置范围为(-w, d.width)
        }
    }
}